Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED includes, in sequence, an anode, an organic EL unit, and a cathode. The organic EL unit disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. “Organic Electroluminescent Diodes”, Applied Physics Letters, 51,913 (1987), and commonly assigned U.S. Pat. No. 4,769,292 demonstrate highly efficient OLEDs using such a layer structure. Subsequently, numerous OLEDs with alternative layer structures have been disclosed. For example, an OLED can have a three-layer organic EL unit that contains an organic light-emitting layer (LEL) between the HTL and the ETL, designated HTL/LEL/ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly includes at least one host material doped with a guest material. Further, there are other multilayer OLEDs that contain more functional layers in the organic EL units. At the same time, many types of EL materials are also synthesized and used in OLEDs. These new device structures and new materials have further resulted in improved device performance.
An OLED is actually a current driven device. Its luminance is proportional to current density, but its lifetime is inversely proportional to current density. In order to achieve high brightness, an OLED has to be operated at a relatively high current density, but this will cause a short operational lifetime. Thus, it is critical to improve the luminous efficiency of an OLED while operating at the lowest possible current density consistent with the intended luminance requirement to increase the lifetime.
In order to dramatically improve luminous efficiency and to increase lifetime in OLED, a tandem OLED (or stacked OLED, or cascaded OLED) structure, can be fabricated by stacking several individual OLEDs vertically and driven by only a single power source. See U.S. Pat. Nos. 6,337,492, 6,107,734, 6,717,358, U.S. Patent Application Publications 2003/0170491 A1, 2003/0189401 A1, and JP Patent Publication 2003045676A. In a tandem OLED having a number of N (N>1) EL units, the luminous efficiency can be N times as high as that of a conventional OLED containing only one EL unit (of course, the drive voltage can also be N times as high as that of the conventional OLED). Therefore, in one aspect to achieve long lifetime, the tandem OLED needs only about 1/N of the current density used in the conventional OLED to obtain the same luminance while the lifetime of the tandem OLED will be about N times that of the conventional OLED. In the other aspect to achieve high luminance, the tandem OLED needs only the same current density used in the conventional OLED to obtain a luminance N times as high as that of the conventional OLED while maintaining about the same lifetime.
In a tandem OLED, all of the EL units are electrically connected in series by inserting an intermediate connector between any adjacent EL units. The intermediate connectors in the tandem OLED play an important role in order to make the tandem OLED useful. The intermediate connectors (or intermediate electrodes) in the prior art tandem OLEDs were mainly formed using metals or other inorganic materials, or combination thereof, such as disclosed in U.S. Pat. Nos. 6,337,492, 6,107,734, and JP Patent Publication 2003045676A. Using a metal or other inorganic material to form an intermediate connector can introduce a fabrication complexity. Many inorganic materials are not easy to deposit by vapor deposition and their deposition methods are sometimes not compatible with the underlying organic layers. In addition, some inorganic materials can cause pixel crosstalk or low optical transparency. Commonly assigned U.S. Patent Application Publication 2003/0170491 A1 and U.S. Pat. No. 6,717,358 teach how to form an intermediate connector (or a connecting unit) by n-type doped organic layer and/or p-type doped organic layer. The doped organic intermediate connector can be formed using only one evaporation method and without causing pixel crosstalk and low optical transparency. Although effective, there is still a need to further improve lifetime, reduce drive voltage and improve manufacturability.